vir] RADIO-ACTIVE EMANATIONS 199 
Since the power of diffusing rapidly through porous substances, 
and acting on a photographic plate, is also possessed by a chemical 
substance like hydrogen peroxide, some experiments were made 
to see if the emanation could be an agent of that character. It was 
found, however, that hydrogen peroxide is not radio-active, and 
that its action on the plate is a purely chemical one, while it 1s 
the radiation from the emanation and not the emanation itself that 
produces ionizing and photographic effects. 
131. Experimental arrangements. The emanation from 
thorium is given off in minute quantity. No appreciable lowering 
of the vacuum is observed when an emanating compound is placed 
in a vacuum tube and no new spectrum lines are observed. 
For an examination of the emanation, an apparatus similar in 
principle to that shown in Fig. 37 is convenient. 
The thorium compound either bare or enclosed in a paper 
envelope was placed in a glass tube C. A current of air from a 
gasometer, after passing through a tube containing cotton-wool to 
remove dust particles, bubbled through sulphuric acid in the vessel 
A. It then passed through a bulb contaming tightly packed 
cotton-wool to prevent any spray being carried over. The emana- 
To Electrometer 

To Battery 
Fig. 37. 
tion, mixed with air, was carried from the vessel C through a plug 
of cotton-wool D, which completely removed all the ions carried with 
the emanation. The latter then passed into a long brass cylinder, 
75 cm. in length and 6 cm. in diameter. The insulated cylinder 
was connected with a battery in the usual way. Three insulated 
electrodes, HL, F', H, of equal lengths, were placed along the axis of 
the cylinder, supported by brass rods passing through ebonite 
corks in the side of the cylinder. The current through the gas, 
due to the presence of the emanation, was measured by means of | 
